1. Field of the Invention
This invention relates to a method of driving a liquid crystal matrix panel having a ferroelectric liquid crystal as a liquid crystal layer.
2. Description of the Prior Art
Recently there has been reports of ferroelectric liquid crystals having a fast response and memory property.
To begin with, a ferroelectric liquid panel known in the prior art will be outlined. A ferroelectric liquid crystal comprises elongated molecules arrayed to form a layer, each being orientated at a specific angle to the layer normal, and has a dipole moment in a perpendicular direction to the longitudinal axis of the molecule and the layer normal. It becomes spontaneously polarized when the layer thickness is made thin. When an electric field is applied to a ferroelectric liquid crystal cell of an extremely small layer thickness, molecules will have two states substantially horizontal to the substrate and orientated at .+-..theta. degrees to the layer normal depending on the direction of application of voltage. By the utilization of this phenomenon and, in addition, electro-optic effect such as birefringence or dichroism, light and shade states can be represented. In a sufficiently-thin-layer cell, the interaction between molecules and the substrate surface ensures that the above-mentioned two states are kept after the electric field has been removed. Such a cell is called a "surface stabilized ferroelectric liquid crystal" (SSFLC). (N. A. Clark, et al., "Submicrosecond bistable electro-optic switching in liquid crystals", Applied Physics Letters Vol. 36(11), pp. 899-901, June 1, 1980)
For driving the ferroelectric liquid crystal panel constructed as above, large amplitude pulses having inverse polarity are required when pixels are turned off as well when as turned on, as distinguished from nematic liquid crystals. On the other hand, like nematic liquid crystals, the time average of the applied voltage must be 0 to prevent the liquid crystal from deteriorating. There has been proposed a time-sharing driving method fulfilling these conditions, which is a slightly modified one from the conventional amplitude selection scheme for nematic crystals. (T. Harada, M. Taguchi, K. Iwasa, M. Kai; SID' 85 Digest (1985), p.131). FIG. 1 shows waveforms of voltages applied to the pixel in the conventional driving method. One scanning consists of two frames which are different in polarity, each frame being based on the 1/4 amplitude selection scheme. Since an ON-state pixel is written in the first frame and an OFF-state pixel is written in the second frame, in the first frame erroneous overlapped representation of the ON-state pixel at the preceding scanning and new data occurs, and this causes display degradation in the case of longer scanning times or animation. Besides, a frequency of at least 30 Hz is necessary for animation and at least several Hz is necessary for an office automation display. In a ferroelectric liquid crystal, the width of one writing pulse is limited by the response time of the liquid crystal. For these reasons, if one scanning requires a number of frames, as in the prior art, then the number of scanning lines representable within the above-mentioned scanning period become lower.
A ferroelectric liquid crystal can be stable only in three states: two where molecules are horizontal to the substrate and the other where molecules having twisted structure between the upper and lower substrates, and thus countermeasures for representing intermediate grey levels are considered: (a) to utilize the complicated state where two or three of the above-mentioned states are mixed in spots, or (b) to vary relative appearance duration of the two or three states with respect to each other. (N. A. Clark et al., "Fast and bistable electro-optic display using ferroelectric liquid crystals", Eurodisplay pp.73-76, 1984). The state mixed in spots is easily affected by the substrate surface, and so it is difficult for a large size panel to obtain a considerable number of grey levels. The method of varying relative appearance duration is superior from the viewpoint of control, but there has not been any study on time sharing driving methods. Furthermore, for the realization by the time sharing driving method, multiple scanning is considered necessary, and this reflects more strict limitations based on the above-mentioned scanning period.